I've listed this in order of fun. I've also included what I found particularly compelling and maybe what might turn you off -
Try some mods: my favorite is Space Exploration, Rampant, and Recipe Randomizer. As of 2023, SE and Rampant have been updated with a lot of incredible mechanics and so worth another look if it has been awhile.
- What I found fun about this - These mods can be extremely challenging and require a lot of fresh innovation and puzzle solving that's barely spoiled anywhere. Recipe Randomizer is super simple, but the great thing about it is your factories will guaranteed to be unique and its near impossible to use prior made blueprints - literally impossible to spoil.
- What you might not find fun about this - they can get very very challenging. In particular, vanilla deathworld rampant is nearly impossible (at least for me)
- Fun: Challenging PvE - you find yourself wanting to try all the toys. As of 2023, there's been a lot of great factorio forum discussions and YouTube videos to check out after you've completed this and see how other folks have shared your pain.
- Not fun: Challenging PvE - for some folks, factorio is about factories and factories only. Fair enough!
- Fun: You can use combinators which can free up the requirement having to put certain items into certain cars. Makes your factories much more flexible. Sometimes planning can be less fun and more work than just stream of conscious factory layouts, so this the best of both world: Optimal throughput and ad hoc factorio.
- Not fun: Trains that never clog or crash are infinitely reliable with huge throughput, but they are a bit of a silver bullet. Sometimes too simple and too reliable can get a bit boring. Best to mix with some of the other mods. Combinators can be quirky and rather necessarily Rube Goldberg at times.
- Fun: You can be fairly optimal and ad hoc about this, just look at your robot enabled map and start with the factories that are flying the most bots. Not really required to optimize the high hanging fruit which doesn't fly a lot of bots and looks more tedious than fun.
- Not fun: Hah! It's all fun and but if part of it isn't, just don't do it. Optimizing is what factorio is all about after all. Well, perhaps sometimes belts haven't been more optimal than bots, sadly, so there's that. That said, by 2023, things have fortunately been tweaked in belt's favor over bots.